As a continuously variable transmission provided in a vehicle, a belt type continuously variable transmission is known. The belt type continuously variable transmission includes a primary pulley to which a driving force of an internal combustion engine is transmitted, a secondary pulley connected to the vehicle wheels, and a belt wound around the two pulleys. The transmission changes a gear ratio continuously in a stepless manner by changing the winding radii of the belt at the pulleys.
In such a belt type continuously variable transmission, the winding radii of the belt at the pulleys are changed by changing hydraulic pressures in hydraulic chambers provided in the pulleys to change thrusts that are forces of the pulleys holding the belt in between, thereby controlling the gear ratio.
More specifically, to reduce the gear ratio, hydraulic pressure in the hydraulic chamber of the primary pulley is increased to increase thrust of the primary pulley, and hydraulic pressure in the hydraulic chamber of the secondary pulley is decreased to reduce thrust of the secondary pulley. This increases the winding radius of the belt at the primary pulley and decreases the winding radius of the belt at the secondary pulley thereby reducing the gear ratio.
To increase the gear ratio, on the other hand, the hydraulic pressure in the hydraulic chamber of the primary pulley is decreased to reduce the thrust of the primary pulley, and the hydraulic pressure in the hydraulic chamber of the secondary pulley is increased to increase the thrust of the secondary pulley. This decreases the winding radius of the belt at the primary pulley and increases the winding radius of the belt at the secondary pulley thereby increasing the gear ratio.
In patent document 1, a controller of a continuously variable transmission is described. In a transient state in which a gear ratio is being changed so that the gear ratio approaches a target gear ratio, the controller calculates the gear shifting speed required for the current gear ratio to conform to the target gear ratio and supplies the hydraulic pressure required to achieve the calculated gear shifting speed to the primary pulley.
The controller of such a continuously variable transmission can execute feedback control based on the difference between the actual gear ratio and the target gear ratio. For example, a basic hydraulic pressure control amount is calculated based on the target gear ratio, an integral term is calculated based on an integral value of the amount of the difference between the current gear ratio and the target gear ratio, and the hydraulic pressure controlled amount is corrected by the integral term is output to control the thrust of the primary pulley.
When such a configuration for controlling the thrusts of the pulleys through the feedback control is employed, it is possible to realize control reflecting a change history of the actual gear ratio.